The invention relates to capacitive pick-off circuits; that is, circuits that are used to determine the capacitance or the gap between plates of a capacitor.
In prior art capacitive pick-off systems, such as described in the copending application of Holdren et al, Ser. No. 702,389, filed on July 6, 1976, assigned to the assignee of this application, capacitance is measured by applying a time varying voltage such as a triangular wave or a saw-tooth waveform to the capacitor plates. The resulting current through the capacitors is then used as a measure of the capacitance or the gap between the plates of the capacitor. However, due to the fact that the capacitance of a capacitor varies in an inverse relationship with the distance or gap between the plates, a change in this gap will result in non-linearities in the output current which can under some circumstances in the application of the capacitive pick-off circuit be significant. For example, a ten percent change in the gap will give rise to over a one percent non-linearity, a twenty percent gap change will give rise to over a four percent non-linearity and a fifty percent gap change will result in a thirty percent non-linearity. Also, since the output current of the capacitor gets very large as the gap becomes very small, significant stability problems can appear when the capacitor is used as a position sensing element in a servoed device. Capacitive pick-off circuits are used in a large number of instrumentation type applications such as transducers and accelerometers and for some highly sensitive instruments such as servoed accelerometers and transducers where non-linearities due to significant changes in the capacitor gap distances can be a significant source of error.
In addition, the prior art capacitive pick-off circuits were, as a practical matter, limited in operation to an information bandwidth less than half of the carrier frequency or the time varying voltage applied to the capacitor plates. Due to the fact that operational amplifiers were most often used in the pick-off circuits the carrier frequency would generally have to be 20 KHz or less.